Hydraulic servomotor system



March 27, 1956 P. G. IPSEN HYDRAULIC SERVOMOTOR SYSTEM Filed April 27,1955 Inventor 1 Peter G. Ipsen His Attcr-ne g United States PatentHYDRAULIC SERVUMDTOR SYSTEM Peter G. Ipsen, Schenectady, N. Y., assignorto General Electric Company, a corporation of New York Application April27, 1955, Serial No. 504,211

5 claims. or. 137-34 This invention relates to a system for controllingthe supply of motive fluid to a turbomachine, particularly to ahydraulic servomotor system which is regulated by oil pressure toactuate the main control valves of a steam turbine.

In the operation of a steam turbine it is desired to have quick openingand closing times of the main control valves. The conventional way ofregulating the main valves of a steam turbine is by a separate oilpressure system which actuates a double-acting servomotor connected tothe main valves to move them to open or closed position. The oilpressure system may include a centrifugal pump which is operated by themain shaft of the steam turbine. The centrifugal pump supplies oil tothe servomotor in accordance with the setting of an intermediate pilotvalve which is controlled by a conventional governor.

With present designs, a skirted piston is used to obtain fast closingwithout requiring such a high rate of oil flow as would be required if askirt was not used. This arrangement is satisfactory since the forcerequired to close the main valves is small relative to that required toopen them, and furthermore, it is feasible to have a valve opening timewhich is appreciably slower than the closing time.

The use of a skirt, however, introduces a problem in upsetting thebalance in the oil fiows in the hydraulic system. This results from thefact that when the servomotor is moved in a direction to open the mainvalves the flow out of the main servomotor is about one-half thatflowing in and the pump inlet pressure falls off considerably. Using askirted piston brings about an opening time on the order of 3 seconds ascontrasted with a closing time on the order of one-half second. Thisarrangement is quite satisfactory on units which utilize only one mainservomotor since reasonable opening times can be obtained withoutserious decrease in the pressure at the inlet side of the centrifugalpump. However, when it is desired to utilize several large servomotorsthere arises the possible danger that it may be diificult to getreasonable valve opening times Without having the pump inlet pressuredrop to zero. An instance of this is that in a certain new doubleautomatic extraction turbine three large servomotors are used, allsupplied with operating liquid from the same pump.

Accordingly, it is an object of this invention to provide an improvedhydraulic system which actuates a valve for controlling the flow ofmotive fluid to a turbine, with desired opening and closing times.

A further object is to provide an improved hydraulic servomotormechanism having a differential displacement which provides fordifferent valve opening and closing times, but which maintains areasonable valve opening time by alternately storing up and supplyingoil to the inlet of the pump supplying operating fluid to theservomotor, so that regardless of the number of servomotors supplied bythe pump,. the pump inlet pressure will not be seriously reduced.

Other objects and advantages will be apparent from the followingdescription taken in connection with the accompanying drawing, whichdiagrammatically illustrates a hydraulic servomotor incorporating theinvention and arranged to operate the main control valves of a steamturbine.

Generally stated, the invention is practiced by providing variablevolume reservoir means associated with the main servomotor. Thereservoir draws in a portion of the oil discharged from the mainservomotor during closing movement of the main valve and returns thestored oil to the inlet side of the pump during valve opening movement,to supplement the oil discharged from the main servomotor so as toprevent the amount of oil available at the inlet side of the pump frombeing reduced to a quantity providing undesirable inlet pressures at thecentrifugal pump, for either opening or closing movements of the mainvalve.

Referring now to the drawing, the invention is disclosed as applied to asteam turbine 1 having a main shaft 2 which drives a centrifugal pump 3.The centrifugal pump 3 supplies oil under pressure to a servomechanism11 which controls the opening and closing of the main steam valve 1athrough a lever mechanism 16, 17 which rotates about pivot 15.

The centrifugal pump 3 is not self-priming and receives oil throughinlet conduit 7 from a pump priming system, described more particularlyin the U. S. Patent 2,440,980 issued May 4, 1948, on an application ofR. Sheppard and assigned to the same assignee as the present invention.

Briefly, the pump priming system comprises a starting motor 20 whichdrives an auxiliary centrifugal pump 21. The pump 21 is located in asump 26 and supplies oil through a conduit 23 and check valve 22 toprovide the required oil for operation of the hydraulic servomotormechanism when the turbine is at a standstill or at such a low speedthat main pump 3 cannot supply the necessary oil. The oil, underpressure, supplied by pump 21 acts on turbine 24 to drive the boosterpump 25 which establishes a pressure at the inlet of the main pump 3through conduit 7. Check valves 4 and 6 prevent oil pumped by theauxiliary pump 21 from flowing back to the oil tank 26 through conduit7.

The hydraulic servomotor 11 is initially actuated by providing oil frompump 21 to the servomotor through conduits 23, 13, 5, pilot valve 8 andline 10. This is permitted by the action of governor 8b. When theturbine is shut down, the governor 8b collapses with the result thatvalve stem 8 is moved up to uncover conduit port 10a so as to receiveoil from line 5 when starting motor 20 is energized. After the turbineis running at rated speed the main pump 3 takes over, forcing checkvalve 22 closed and the auxiliary pump 21 may be shut down. Thisstarting operation is noted merely to give a more complete picture ofthe system, but is not essential to an understanding of the presentinvention.

The centrifugal pump 3 provides oil under pressure to the main hydraulicservomotor 11 through conduit 5, check valve 4-, pilot valve 8, andconduits 9 or 14) depending on whether the main servomotor is to beactuated in a direction to move the valve 1a to open or closed position.

To control the flow of oil between conduits 5 and conduits 9 or 10, apilot valve 8 is used. The pilot valve 8 is a conventional piston valvehaving a series of openings adapted to receive conduits 5, 9, 10, 14a,and 14b. Within the pilot valve 8 is a double piston arrangement 8d, 3ewhich pistons are axially disposed on rod 8 The pistons 8d, 8e arelocated to control the flow of oil at ports 901, 10a respectively. Whenrod 8 is moved downwardly, conduit 5 is connected with line 9 andconduit 10 with line 1411'. In this position, the oil from centrifugalpump 3 acts on the servomotor 11 to move the main valves to a closedposition. At the same time the conduits and 1412 are interconnected tovent the oil discharged from the servomotor 11 back to the inlet ofcentrifugal pump 3. if the pilot valve 8 is moved in the upwarddirection the conduits 5, 10 would be interconnected to supply oil tothe servomotor to move it in the direction to open the valves 1a. Theoil discharged from the servomotor would then be discharged throughlines 9, 14a back to the inlet of centrifugal pump 3.

The pilot valve is regulated in the conventional manner by a governor 8bwhich operates in accordance with the speed of the main turbine shaft 2through gears 8a. This method of operation is conventional and is citedby way of example only.

The main scrvomotor 11 comprises a. housing 11g containing therein askirted piston 11b. Piston 11b divides the housing 11g into chambers11a, 11c. Extending from the opposite sides of the piston 11!): are rods1111, 111'. The diameters of the piston rods 11h, 11i are very substantially different, so the effective area of piston 11b subject to theoil pressure in chamber 11a is substantially one-half the effective areaof piston 11!) which is subject to the pressure in chamber 110. Thisratio between the effective areas on the opposite sides of piston 11!)is by way of example only, since it will be understood by those skilledin the art that this ratio can be varied as selected by the designers tosecure a desired relation between the times required for opening andclosing the valve 1a.

In accordance with the invention, the main servornotor defines asupplementary casing 11d adjacent the bottom portion of casing Hg. Thesmall diameter rod 111' extends through the bottom of casing 11g and topof casing 114. The chamber 11 is formed between the casing 11d andpiston 11:: secured to the rod 112'. The effective area of piston 11::is equal to the difference between the effective lower and upper areasof piston 1112. Thus the sum of the upper effective area of pistons 11!)and 11e (in chambers 11a, 11 is equal to the lower elfective area ofpiston 11!) (in chamber 110).

The chamber 11] is connected with the inlet 7 of centrifugal pump 3through conduit 14c. Conduit 140 is also connected to conduits 14a and14b for reasons to be described later.

The function of hydraulic servomotor 11 is to move main valve In to aclosed or open position in response to the oil pressure in the chambers11a and 11c respectively.

The operation of the hydraulic servomotor system is as follows.

The starting motor is energized to operate the centrifugal pump 21disposed in the oil sump 26. Oil pressure is thus supplied through checkvalve 22, conduits 23, 13, and 5, pilot valve 8, conduit 10 and intochamber 110. This takes place when the governor 8b is collapsed touncover port 16a. The oil pressure in chamber 110 moves piston 11b inthe upward direction to open valve In to admit steam to the turbine 1.The pump 21 also provides oil pressure through conduit 23 to drive thehydraulic turbine rotor 24. The turbine rotor in turn drives the primingpump 25 which supplies oil under pressure to the inlet of pump 3 throughconduit 7.

When the turbine 1 is started, it drives the centrifugal pump 3 throughthe drive shaft 2. Part of the output of the pump flows through conduits13 and 23 to drive the turbine 24. When this happens, the starting motor20 is shut off.

The governor 8b is controlled by the speed of shaft 2 to regulate thepilot valve 8 which controls the oil flowing from conduit 5 to eitherconduits 9 or 10. The speed of the shaft 2 is controlled in accordancewith the operating requirements of the turbine.

When, in response to an operating condition of the turbine, less steamis required the following takes place:

The governor 8b, in response to the increased speed of shaft 2, actuatesthe pilot valve 8 in the downward direction. This downward movement ofthe pilot valve 8 interconnects the conduits 5 with 9 and 10 with 1412.The oil discharged by the centrifugal pump 3 then flows through checkvalve 4, conduit 5, pilot valve 8, conduit 9 and finally into servomotorchamber 11a where it acts on piston 11]) to move it in a downwarddirection. This downward motion of piston 11b acts through linkage 16,i7, which pivot about fulcrum 15 to move valve 1a in the closingdirection. Due to the ratio between the eliective areas on the oppositesides of piston 11b, the downward movement of the piston 11b displacestwice the amount of oil from chamber than is received in chamber 1111.This oil discharged from chamber 11c flows through conduit 10, pilotvalve 8 and into conduit 1%.

The downward movement of piston 11b also caused piston lie to enlargethe size of chamber 11 This draws oil from conduit 14b through conduitinto chamber 11f. Since the effective area of piston 112 is equal to thedifference between the effective areas on the opposite sides of piston1112, the chamber 11 will be enlarged that amount necessary to store thediifercnce between the oil displaced from the chamber 110 and thatfiOV-Jillg into chamber 11a. The balance will flow through conduits 14and 7 to the inlet side of pump 3.

When the pilot valve 8 is moved in an upward direction the followingtakes place. The oil is pumped from centrifugal pump 3 through conduit5, pilot valve 8, conduit it? and into the chamber 11c. This buildup ofpressure in chamber 11c tends to move piston 11b in an upward directionto move the valve In in the opening direction. The oil in chamber 11a isforced out into conduit 9 and'flows through pilot valve 8 into conduit14a. Since the amount of oil flowing out of chamber 11a is substantiallyequal to one-half that flowing into chamber 11c (due to the differencebetween the effective areas defined by the respective sides of piston11b) it can be seen that the amount of oil available at the inlet sideof centrifugal pump 3 would be diminished by the difference between theoil flowing in conduits 9 and 10 unless a supplementary source of oil isprovided. Any such reduction in the amount of oil available would reducethe inlet pressure available to pump 3. If there were a number ofservomotors all being supplied by the same pump, the inlet pressurecould drop to zero, which would, of course, seriously affect theoperation of the centrifugal type pump.

This supplementary source of oil is provided by the reservoir 11attached to the main servomotor. When the piston 11b moves in an upwarddirection it carries along with it through rod Hi the piston He. Thepiston 11e discharges the stored up oilin chamber 11 into conduit 140.The size of the piston lle is such that the amount of oil discharged toconduit 14 is equal to the difference between the oil flowing into andout of servomotor 11. Thus, the amount of oil flowing to the inlet sideof centrifugal pump 3 is substantially equal to the amount of oil pumpedthrough conduit 5 to the chamher 110. The inlet pressure of thecentrifugal pump is thereby maintained substantially constant.

Thus it can be seen that with the aid of this invention, the quantity ofoil available at the inlet side of the main pump 3 is maintainedsubstantially constant regardless of the munber of servomotors suppliedby the centrifugal pump 3. This prevents the inlet pressure ofcentrifugal pump 3 from dropping off substantially.

While a single embodiment of the invention has been described in detailherein, it will be obvious to those skilled in the art that many changesand substitutions of equivalents might be made. For example, it may bedesirable to make the upper area of piston 11e somewhat less than thedifference between the upper and lower areas of piston 11b. This can bedone to achieve a better balance regarding the drop in main pump inletpressure during opening and closing strokes of the servomotor.

This drop in pressure will occur because the extra flow delivered bymain pump 3 during motion of the servomotor can be obtained only atsomewhat reduced hydraulic pressure, due to the centrifugal pumps flowcharacteristics. This reduces the available energy to the oil turbine24; pump 25 will slow down; and the pressure in conduit 7 will tend todrop. This action can be counteracted to any desired extent for openingor closing stroke (not both) by returning more oil to the main pumpinlet than is taken from it by the servomotor 11. Since the maintenanceof very rapid closing time is of prime importance, this can be done byreducing the displacement of piston lie. Doing this, however, willincrease the minimum possible opening time because on opening there willbe less oil returned to the main pump than is taken from it.Considerable latitude exists for moving in this direction, becauseopening time can be perhaps anywhere from 6 to 10 times the closingtime.

It is, of course, desired to cover by the appended claims all suchmodifications as fall Within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A hydraulic system comprising a servomotor, a pump for supplyingoperating fluid under pressure to the servomotor, a pilot valve forcontrolling the flow between the pump and the servomotor, the servomotorcomprising a first housing, a first piston means defining two differenteflfective areas with said first housing, a first chamber definedbetween the first housing and the larger of the two areas, a secondchamber defined between the first housing and the smaller of the twoareas, a second housing defining therein a third chamber between thesecond housing and a second piston located in said second housing, meansconnecting the second piston to said first piston whereby when the firstpiston is moved in a direction to decrease the size of the first chamberthe second and third chambers are increased in size, and conduit meansinterconnecting the pump, pilot valve, and said first and secondhousings whereby, upon movement of the pilot valve to direct the flow offluid from the pump to the second chamber, the first piston will move ina direction tending to displace fluid from the first chamber to theinlet of the pump, the second piston will move to enlarge the thirdchamber into which fluid not recirculated by the pump is directed andstored, and, upon movement of the pilot valve in the opposite direction,fluid is supplied to the first chamber and the first piston displacesfluid from the second chamber to the pump inlet and the second pistondisplaces fluid from said third chamber to the inlet of the pump toprevent the pump inlet pressure from substantially dropping.

2. Apparatus of the character described which includes a main pumpsupplying fluid under pressure through first conduit means to aservomotor, pilot valve means for regulating the flow between the pumpand the servomotor, second conduit means connecting the pilot valve withthe inlet of the pump, the servomotor consisting of a first and secondhousing, first piston means defining two diiferent effective areas withsaid first housing and dividing the first housing into a first chamberdefined between the first housing and the larger of the two areas and asecond chamber defined between the housing and the smaller of the twoareas, each being adapted to receive fluid through said first conduitmeans as determined by the position of the pilot valve, the secondhousing having a second piston defining a third chamber of variablevolume, means connecting said second piston to move with said firstpiston, the second piston having an effective area exposed to thepressure in said third cham ber equal to the difference between theeffective areas on the opposite sides of the first piston, the pistonsbeing so constructed and arranged that movement of the first piston in adirection to decrease the size of the first chamber increases the sizeof said second and third chambers,

third conduit means connecting the third chamber with the second conduitmeans, whereby, when the pilot valve is positioned to supply operatingfluid to the second chamber, excess fluid discharged from the firstchamber and not recirculated by the pump is stored in said thirdchamber, and, upon movement of the pilot valve to supply fluid to thefirst chamber, the fluid stored in said third chamber is discharged tothe inlet of the pump to prevent the pump inlet pressure fromsubstantially decreasing.

3. A hydraulic system for actuating a control valve for supplying motivefluid to a turbine, the combination of a main pump located at theturbine and arranged to be driven thereby, a servomotor connected toactuate the control valve and comprising a first housing, first pistonmeans defining two different eflective areas with said first housing, afirst chamber defined between the first housing and the larger of thetwo areas, a second chamber defined between the first housing and thesmaller of the two areas, a second housing defining therein a thirdchamber between the second housing and a second piston located in saidsecond housing, means connecting the second piston to said first pistonwhereby when the first piston is moved in a direction to decrease thesize of the first chamber the second and third chambers are increased insize, a pilot valve controlling the flow between the pump and theservomotor, first conduit means connecting the outlet of the pump withthe pilot valve, second and third conduits means connecting the pilotvalve with the first and second chambers of the servomotor respectively,fourth conduit means connecting the pilot valve with the inlet of saidpump, fifth conduit means connecting the third chamber with the inlet ofsaid pump, the pilot valve containing valve means controlling fluid flowbetween the first, second, third, and fourth conduit means whereby, uponmovement of the valve means to direct the flow of fluid from the firstconduit means to the third conduit means and the second chamber, thefluid discharged from the first chamber due to the movement of the firstpiston flows through the second and fourth conduit means to the inlet ofthe pump and the fluid not recirculated by the pump flows through thefifth conduit means to the third chamber which is enlarged by themovement of the second piston where it is stored, and upon movement ofthe valve means in a position to interconnect the first conduit meanswith the second conduit means to supply fluid to the first chamber whichactuates the first piston means to discharge fluid from the secondchamber, the fluid discharged from the second chamber flows through thethird and fourth conduit means to the inlet of the pump and the fluidstored in the third chamber is discharged therefrom by the action of thesecond piston means to the inlet of the pump to prevent the pump inletpressure from substantially dropping off.

4. The hydraulic system comprising a servomotor, a pump for supplyingoperating fluid under pressure to the servomotor, the servomotorcomprising a first housing containing therein first piston meansdefining two difiterent effective areas, a first chamber defined betweenthe first housing and the larger of the two areas, a second chamberdefined between the first housing and the smaller of the two areas, asecond housing defining therein a third chamber between the secondhousing and a second piston, means connecting the second piston to saidfirst piston whereby when the first piston is moved in a direction todecrease the size of the first chamber the second and third chambers areincreased in size, a pilot valve for controlling the flow between thepump and the servomotor, the pilot valve including a housing defining anaxial bore and a pressure fluid inlet port, drain ports spaced axiallyon either side of the inlet port, outlet ports defined between the inletport and each of said drain ports, and a reciprocating flow controlmember disposed in said bore and comprising an axially slidable memberWith at least two axially spaced lands disposed in said bore incooperation with said outlet ports, a first conduit means connecting theoutlet of the pump with said inlet port, second and third outlet meansconnecting said outlet ports with the first and second chambers of saidservomotor respectively, fourth conduit means connecting the drain portswith the inlet of said pump, fifth conduit means connecting the thirdchamber with the inlet of said pump, whereby upon movement of the flowcontrol member to a position which interconnects the first conduit meanswith the third conduit means that fluid will be supplied from said pumpto said second chamber and the fluid discharged from said first chamberby the movement of the first piston means will be supplied to the inletof the pump to be recirculated, the excess fiuid being stored in thethird chamber which increases in size upon decrease of said firstchamber, and upon movement of the flow control member in the oppositedirection the inlet port is connected with said second conduits means tosupply fluid to said first chamber and the fluid discharged from saidsecond chamber is supplied to the inlet of the pump and is supplementedby the fluid discharged from said third chamber due to the movement ofsaid first and second pistons in a direction tending to decrease thesize of said second and third chambers, whereby the pump inlet pressureis prevented from substantially dropping.

5. in a hydraulic servomo-tor system for actuating a control valve forsupplying motive fluid to a turbine, the combination of a main pumplocated at the turbine and arranged to be driven thereby, auxiliarypriming pump means for supplying fluid under pressure through firstconduit means to the inlet of the main pump, a servomotor connected toactuate the turbine control valve, second conduit means for supplyingfluid from the main pump to a pilot valve which regulates the flow ofoperating fiuid to the serve-motor, the pilot valve including a housingdefining an axial bore and a pressure fluid inlet port adapted toreceive said second conduit means, the pilot valve housing also havingdrain ports spaced axially on either side of the inlet port, outletports defined between the inlet port and each of said drain ports, and areciprocating flow control member disposed in said bore and comprisingan axially slidable member with at least two axially spaced landsdisposed in said bore in cooperation with said outlet ports, theservomotor comprising a first housing, first piston means defining twodifierent effective areas with said first housing, a first chamberdefined between the first housing and the larger of the two areas, asecond chamber defined between the first housing and the smaller of thetwo areas, third and fourth conduit means connecting one of the pilotvalve outlet ports with said first and second chambers respectively, andauxiliary reservoir means including a second housing containing a pistonconnected to move with said first piston, a third chamber definedbetween said second piston and the second housing, the sum of theeffective areas of the pistons in the second and third chambers beingsubstantially equal to the efiective area of the first piston in thefirst chamber, the pistons being so constructed and arranged that whenthe first chamber decreases in size the second and third chambersincrease, fifth conduit means connecting the pilot valve drain portswith said first conduit means, and sixth conduit means connecting thethird chamber with said first conduit means, whereby, upon movement ofthe first piston in the direction tending to discharge fluid from thefirst chamber, excess fiuid over that recirculated by the main pump isstored in the third chamber, and upon movement of the first piston inthe opposite direction the fluid stored in said third chamber isdischarged to the inlet side of the main 1 pump to prevent the pumpinlet pressure from substantially dropping.

References Cited in the file of this patent UNITED STATES PATENTS1,468,595 Ferris Sept. 18, 1923 2,497,608 Herrstrum et al. Feb. 14, 19502,699,651 Douglas Jan. 18, 1955

